1. Field of Invention
This Invention relates to chemical hydrolysis processing of renewable lignocellulosic biomass inorder to produce a single solution of sugars and a solid lignin-residue.
2. Description of Prior Art
Here-to-fore, a plug-flow-reactor has been proposed to try to gain higher hydrolysis-conversion of cellulose to glucose, by using extremely-high hydrolysis-rates, achieved by high-temperatures of reaction, as provided by direct-injection of high-pressure steam into high-solids density slurries.
Even-so, hemicellulose hydrolysate sugars are all degraded, by such a single-stage high-temperature polysaccharide hydrolysis reaction process. The hydrolysate-sugars high-dilution by steam condensation causes the resulting single-solution of glucose to have a low-concentration and large-volume with high-cost acid-neutralization, and inefficient fermentation.
U.S. Pat. No. 4,201,596 to Church (1980) shows a continuous process for effecting the acid-hydrolysis of cellulosic waste materials in high-solids density slurries. By control of high temperature, through direct steam injection, the high density slurry solids may be converted to yields of about fifty percent of the potential glucose in cellulose in seconds. This chemical processing method, for converting polysaccharides into pentose and/or hexose sugars, is by a known use of a tubular-type plug-flow-reactor (PFR) for dilute-acid cellulose hydrolysis. Unfortunately, relatively low conversions, negative byproduct formations, high energy-requirements and impractical high-density slurry-pumping to pressure over 500 psi, have limited the commercial use of that cellulose-conversion by PFR method to R&D investigations.
Researchers, at Dartmouth College in 1978-79, were investigating the acid-hydrolysis of municipal refuse and other materials, using a plug-flow-reactor. The Dartmouth research work involved a 1.3-cm (0.5-in.) diameter plug-flow-reactor, operated at temperatures up to 260.degree. C./500.degree. F., with a variety of biomass materials, including mixed hard-wood, newsprint and corn stover; using a feedstock-rate of 120 pounds per day, and a plug-flowreactor (PFR) for dilute-acid, ligno-cellulosic hydrolysis, at solids concentrations up to 13.5 wt %. The system was a continuous-flow electrically-heated tubular reactor.
The Dartmouth research hydrolysis was flashed through an orifice to stop the reaction at residence detention times of 5-30 s., then cooled. Glucose yields from hard-wood flour ranged as high as 55% in 1983. This work showed that high yields were obtainable on a small scale; several operational problems were encountered that were difficult to solve on a small bench-scale system. Problems included tar build-up and rapid plugging of the small-diameter reactor prevented long runs from being conducted to obtain extensive operating experience.
The Dartmouth Process is known to have the following characteristics: 1.) relatively high-density slurries are very difficult to be pumped at high-rates to high-pressures and through the PFR, 2.) very high reaction-temperatures, on the order of 260.degree. C./500.degree. F., require up to 600 psi steam and 3.) very short hydrolysis reaction-times, of fractions of one-minute, for flow through the PFR, generally.
U.S. Pat. No. 4,615,742 of Wright (1986) shows a processing batch percolation-type hydrolysis reactor. In this counter-current hydrolysis, a flow of dilute-acid solution contacts a body of particulate wood which is moving in a direction opposite to the flow of the dilute-acid solution. The counter-current flow of the dilute-acid solution and the particulate wood results in: a much higher yield of sugars from the wood, a minimal degradation and a relatively high concentration of glucose, but the process conditions result in a low xylose in the dilute-acid hydrolysate solution.
The primary dis-advantage of this particular approach, for counter-current hydrolysis, is the extreme mechanical complexity and expense of moving by conveying the solids and pumping the liquids in the opposite directions.
U.S. Pat. No. 4,612,286 of Sherman (1986) shows a method conceived in an attempt to solve the above problems and provide an approximate counter-current flow processing, without the necessity for actual movement of the wood particles. In general, a plurality of Kamyr percolation hydrolysis reactors are piped together, in-series. This method utilizes a counter-current diffusional treatment structure. Cellulose hydrolysis is practiced in upright diffusion vessel with counter-current flow.
U.S. Pat. No. 4,070,232 of Funk (1978) has found that yield and operability are improved by conducting a ligno-cellulose pre-hydrolysis first and then a hydrolysis of the residue. By pre-hydrolysis of the fresh feedstock, at below 150.degree. C., the hemi-cellulose can be hydrolyzed at temperatures where sugar degradation is relatively insignificant. This allows high yields and recovery, by separation of sugars from hemi-cellulose hydrolysis. It also opens up the structure of the wood particules so that: infusion of acid and diffusion of cellulose hydrolysate-sugars are enhanced, minimum fouling in the pipes by tars and limited degradation products.
The present invention's moderate solids-density slurries and moderate hydrolysis-reaction temperatures and the improvement for recycle of a fraction of unhydrolyzed alpha cellulose residue in stage-two, provides about 65% cellulose to glucose conversion, compared to about 50% for known processes. In addition, other process improvements in the present invention result in over two-times higher hydrolysate-sugar concentrations in a single-solution of two-hydrolysates product, than that of known single solution processes.